it's always been there (at least that's what i remember when i looked into it before getting the NV3550)
if you go that route don't get a flip kit, just flip it with a rail and shifter kit from AA (any D300 would have the rails pitted, fabrication required for the shifters to work or use cable shifters) and buy National seals for the rails (the Omix ones suck, got Nationals this time around and doesn't leak one bit). You'll need the JB conversion spring/ball kit or use 11/32 balls and shim the stock springs (that's what i did).
Depending how much you pay for it might be worth getting an atlas from the get-go
(what i listed is from memory)
D300 t-case ~ $150
AA shifter rail kit ~ $100
D300 Gasket, Seal and shim kit ~ $50
D300 Bearings, shaft & needle kit ~ $75
D300 21 spline Input $155
Cable shifters ~ $200 (guessing, don't know for sure)
JB LoMax gears = $575
JB spring/ball kit = $35
National Shifter rail seals $8
Shipping for all the above let's say $200
Time and frustration - priceless
Total: $1548 or so (if i added them up correctly in my head)
didn't add the clocking ring, that's another at least $50
you're probably better off with an Atlas already setup for 21 spline with 4:1 gears, dunno if they come with shifters but if they do should be a no-brainer imo.